FM receivers are used to receive desired carrier signals in narrow frequency bands. Such receivers typically include a static filter that filters out all but a predetermined band of desired frequencies. Automatic Frequency Control (AFC) systems are known, which improve sensitivity in radios having modest frequency stability performance. In AFC systems, the output of a receiver discriminator is low pass filtered at a very low frequency. The output of the filter is then driven to zero volts by adjusting the frequency of a local oscillator. An advantage of this system is that there is no ambiguity about which way to adjust the frequency of the local oscillator to compensate for the frequency offset and interference. In these systems, the low pass filtered discriminator output is proportional, in both magnitude and sign, to the local oscillator adjustment required.
AFC systems operate under the assumption that there is negligible low frequency content in the desired signal. The AFC systems also operate with the assumption that centering the signal maximizes the signal-to-noise ratio. These assumptions work well for 25 kHz and 30 kHz channel land-mobile radio systems where adjacent channel interference is negligible. In these systems, adjacent channel interference protection levels of 80 dB or more are common.
Increasingly, however, land mobile radio users are requiring spectrally efficient high-speed digital systems. In response, one standards setting body is initiating a 9.6 kbps, 12.5 kHz channel system proposal and the Federal Communications Commission is splitting 25 kHz and 30 kHz channels into 12.5 kHz channels. As this narrow-banding occurs, it reduces the adjacent channel interference protection levels by 20 dB or more. Thus, in these 12.5 kHz channel systems, substantial adjacent channel interference is present. This interference is expected to degrade coverage in some frequency bands and geographical areas, as compared to what is currently available.
As a result of the increase in adjacent channel interference present when narrow-banding, the approach of minimizing frequency offset is not necessarily the most beneficial. In fact, in some interference cases it is better to adjust the receiver away from the desired signal. In these cases, the sensitivity degradation is minimized at a frequency in the opposite direction from the desired transmission. There is no way in the prior art to utilize this signal improvement technique (i.e., adjusting the receiver away from the desired signal).
Thus, a need exists to improve received signal quality in a narrow-band communication system by reducing adjacent channel interference.